I don't know why people think "Blaine" was a fan mistranslation. If anything, "Brain" was an SENA mistranslation. The "l" is replace with "r" in Japanese-spoken English words because that's the style. If the original was "Bu-rain-u" or something like that, then it would be natural that the English translation was "Blaine". Again, "Brain" was probably another SENA translation error (right Roxas).

Maybe they can be both correct translations, because in japanese Blaine and Brain sound the same? I don't know japanese, I'm just guessing, but if this is the case then there's no mistake, just a choice they made.

Kingdom Hearts 15th Anniversary limited merchandise is available for pre-order from 7Net in Japan

To further celebrate the 15th anniversary of Kingdom Hearts this year, 7Net is offering exclusive Kingdom Hearts merchandise from Japan. All items are currently available for pre-order and are releasing on a variety of dates, ranging from June 23 2017 to August 23 2017. You can see some images of the available products for sale below:

A list of release dates and prices of items available from 7Net are as follows: